Process for producing fine metal oxide particles

ABSTRACT

Disclosed herein is a process for producing fine metal oxide particles comprising the step of heat-treating an aqueous solution of a metal salt at a temperature of not lower than 200° C. under a pressure of not less than 160 kg/cm 2  for 1 second to 1 hour so as to bring into the decomposition reaction of said metal salt.

This is a division of application Ser. No 07/714,225 filed on Jun. 12,1991, now U.S. Pat. No. 5,480,630.

BACKGROUND OF THE INVENTION

The present invention relates to a process for producing fine metaloxide particles and, more particularly, to a process for producing finemetal oxide particles by heat-treating an aqueous solution of a metalsalt under the subcritical to supercritical conditions for water.

The fine metal oxide particles are used in various fields, for example,as a material of ceramics, electronic devices, catalysts, cosmetics andcoatings, reinforcing materials and fillers for modifiers, and variousmethods of producing the fine metal oxide particles are conventionallyknown.

The fine metal oxide particles used in the above fields are required tohave a narrow particle size distribution width and the uniform shape, tobe free from a dendrite or a crystal twinning, and to have few secondaryagglomerates. The fine metal oxide particles are sometimes required tobe uniformly agglomerated or amorphous to a great extent.

Such fine metal oxide particles are conventionally produced bycoprecipitation method, heat hydrolysis method or autoclaving method. Inthe coprecipitation method, the particle size is greatly influenced bythe temperature, concentration, charging ratio, charging rate, method ofsupplying the raw materials, and stirring, so that the particle diameteris not always uniform, resulting in a wide particle size distributionwidth. To prevent this, various means are taken such as the addition ofvarious additives and the aging of the fine particles after thecompletion of the reaction.

In the heat hydrolysis method and the autoclaving method, an autoclaveis used and a comparatively long reaction time or aging time such as 30minutes to 48 hours is necessary.

That is, no process for producing the fine metal oxide particles withhigh productive efficiency has not been established.

As a result of various investigations by the present inventors, it hasbeen found that by heat-treating an aqueous solution of a metal saltunder the subcritical to supercritical conditions for water fine metaloxide particles having excellent characteristics such as narrow particlesize distribution width and uniform particle shape can be obtained in ashort time. The present invention has been achieved on the basis of thisfinding.

SUMMARY OF THE INVENTION

In an aspect of the present invention, there is provided a process forproducing fine metal oxide particles comprising the step ofheat-treating an aqueous solution of a metal salt under a pressure ofnot less than 160 kg/cm² and at a temperature of not lower than 200° C.to achieve a decomposition reaction of the metal salt.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an electron micrograph of the particle structure of theboehmite obtained in Example 1;

FIG. 2 is an electron micrograph of the particle structure of theboehmite obtained in Example 3;

FIG. 3 is an electron micrograph of the particle structure of theboehmite obtained in Example 5;

FIG. 4 is an electron micrograph of the particle structure of the α--Fe₂O₃ obtained in Example 7;

FIG. 5 s an electron micrograph of the particle structure of the --Fe₂O₃ obtained in Example 8;

FIG. 6 is an electron micrograph of the particle structure of the Fe₃ O₄obtained in Example 10; and

FIG. 7 is an electron micrograph of the particle structure of the Fe₃ O₄obtained in Example 11.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The metal salt used in the present invention is not specified so long asit is water-soluble. For example, the metal salts of the metals of IB,IIA, IIB, IIIA, IIIB, IVA, IVB, VA, VB, VIB and VIIB groups andtransition metals are usable. More concretely, inorganic acid salts suchas nitrates, chlorides, sulfates, oxyhydrochlorides, phosphates,borates, sulfites, fluorides and oxyacid salts of Cu, Ba, Ca, Zn, A1, Y,Si, Sn, Zr, Ti, Sb, V, Cr, Mn, Fe, Co and Ni, and organic acid saltssuch as formates, acetates, citrates, oxalates and lactates of Cu, Ba,Ca, Zn, A1, Y, Si, Sn, Zr, Ti, Sb, V, Cr, Mn, Fe, Co and Ni may beexemplified. These metal salts may be used in the form of a mixture. Thecomplexes of these metals are also usable. If the corrosion of thematerial of the apparatus or the like is taken into consideration,nitrates are preferable.

The concentration of the metal salt in the aqueous solution does notspecified so long as it is dissolved, but the preferred metal saltconcentration is 0.0001 to 1 mol/1.

A batch-type reaction process or a semi-batch-type reaction process isusable, but a continuous reaction process which charges the stocksolution in a tubular

is preferable. The reaction temperature and the reaction pressure is notless than 200° C. and not less than 160 kg/cm², respectively, which arethe subcritical or supercritical conditions for water. The preferablereaction temperature is 300 to 500° C. and the preferable reactionpressure is 250 to 500 kg/cm². The reaction is especially preferablycarried out under the supercritical conditions for water.

The reaction time in the reactor is different depending upon thereaction conditions, but ordinarily 1 second to 1 hour, preferably 1 to10 minutes.

The reaction product is collected by cooling it before the reactionpressure is reduced and (i) causing it to flow out to aconstant-pressure system at a constant speed through a reducing valve or(ii) reducing directly the reaction pressure to the ordinary pressure.

In the present invention, an alkali and/or acidic aqueous solution maybe added to an aqueous solution of a metal salt. As examples of thealkali may be exemplified ammonia, sodium hydroxide and potassiumhydroxide, and as examples of the acid may be exemplified inorganicacids such as sulfuric acid, nitric acid, hydrochloric acid, carbonicacid and phosphoric acid, and organic acids such as formic acid, aceticacid, citric acid, oxalic acid and lactic acid.

In the present invention, a reducing gas such as hydrogen or an acidicgas such as oxygen may be introduced into an aqueous solution of a metalsalt.

In order to separate the target fine metal oxide particles from thethus-obtained reaction product, a known method may be adopted. Forexample, the separation may be carried out by filtration, centrifugalseparation, spray drying or ultrafiltration. It is also possible todirectly obtain the dried fine metal oxide particles by reducingdirectly the reaction pressure to the ordinary pressure without coolingthe reaction product.

Under the subcritical to supercritical conditions for water, thedissociation of water is enhanced to a great extent, and water acts onthe aqueous solution of the metal salt as a stronger acid, therebygreatly facilitating the progress of the hydrolysis reaction or themetathetical reaction of the metal salt. In addition, since the physicalproperties such as the ionic product, dielectric constant, diffusionrate and thermal conductivity of a fluid greatly vary under thesubcritical to supercritical conditions, it is easy to control the shapeand the size of the fine metal oxide particles by controlling thereaction route and the reaction rate. It is possible to make theheat-transfer coefficient larger under the subcritical to supercriticalconditions than in a gaseous phase and the diffusion coefficient muchlarger than in a liquid phase. It is, therefore, possible to make thereaction conditions in the reactor uniform in a wide range including amicro size at a molecular level, thereby suppressing the increase in theparticle diameter distribution width and the scatter in the particleshape which are derived from the scatter in temperature gradient,pressure gradient or concentration.

As described above, according to the present invention, it is possibleto finish the reaction in a short time and to further easily obtain thefine metal oxide particles having uniform particle diameter and shape byvarying the reaction conditions such as the concentration of the rawmaterial, addition of an alkali or an acid, introduction of a reducinggas or an acidic gas, the reaction temperature and the reactionpressure.

The shape of the fine particles of a metal oxide obtained by the processof the present invention is spindle-like, acicular, scale-like,hexagonal plate-like, square plate, ellipsoid, sphere, octahedron, orthe like. The shape can be selected by varying the reaction conditions.Namely, the fine metal oxide particles obtained having a narrow particlesize distribution width [for example, particle size distribution width=(0.7-1.3) × average particle size] in the particle size range of amajor axial diameter of 100 to 2000 nm and a minor axial diameter of 50to 1000 nm in the case of spindle-shaped particles; a major axialdiameter of 100 to 1000 nm and a minor axial diameter of 20 to 200 nm inthe case of acicular particles; a plane diameter of 100 to 2000 nm inthe case of scale-like particles; a plate diameter of 100 to 1000 nm inthe case of hexagonal plate-like particles; a side 50 to 1000 nm long inthe case of square plate-like particles; a major axial diameter of 30 to300 nm and a minor axial diameter of 20 to 200 nm in the case ofellipsoidal particles; a particle diameter of 20 to 500 nm in the caseof spherical particles; and a particle diameter of 20 to 1000 nm in thecase of octahedral particles.

Accordingly, the fine metal oxide particles produced by the process ofthe present invention are useful as a raw material of ceramics,electronic devices, catalysts, cosmetics and coatings, reinforcingmaterials and fillers for modifiers.

EXAMPLES

The present invention will be explained in more detail hereinunder withreference to the following examples.

In the examples, the reacting apparatuses A and B which will bedescribed in the following were used.

(Reaction apparatus A)

The suction portion of a pressure pump is introduced into an aqueoussolution of the raw material through a pipe, and the discharging portionof the pump is connected to a stainless steel pipe 3.18 mm in outerdiameter and 1.85 mm in inner diameter which is coiled to a length of 12m. This reactor is placed in a heating furnace and a cooling portion isprovided at the exit of the heating portion. A reducing valve isprovided so that the reaction product is collected to a collector at aconstant pressure.

(Reaction apparatus B)

An apparatus similar to the reaction apparatus A except that the reactorin apparatus A is replaced by a stainless pipe having an outer diameterof 9.53 mm, an inner diameter of 6.23 mm, and a length of 400 mm.

Example 1

An aqueous solution of aluminum nitrate having a concentration of 0.01mol/1 was charged into the reaction apparatus A and brought into adecomposition reaction at a flow rate of 4 ml/min at a temperature of350° C. under a pressure of 300 kg/cm². The reaction product obtainedwas a cloudy aqueous solution. From the X-ray analysis of the powderobtained by filtering and drying the reaction product, it was observedthat 100% boehmite (AlOOH) had been produced. The uniformlyspindle-shaped particles having a particle diameter of 1500×700 nm wereobserved in an electron micrograph, as shown in FIG. 1.

Examples 2 to 12

Aqueous solution of metal salts shown in Table 1 were brought into adiscomposition reaction by using the reaction apparatus A or B under theconditions shown in Table 1 in the same way as in Example 1. Thereaction products were analyzed with X-rays and the shapes of theparticles were observed in electron micrographs. The typical examples ofthe electron micrograph are shown in FIGS. 2 to 7.

As shown in Table 1, boehmite particles varying in shape and size wereobtained from aluminum nitrate, as shown in Table 1.

In the case of using an iron salt, various substances were producedunder the influence of anions and cations. The lower the concentrationof the raw material, the finer were the particles obtained.

                                      TABLE 1                                     __________________________________________________________________________             Reaction conditions                                                  Reac-                    Press-                                                                            Flow                                                                              Reaction products                                 tion      Concent-                                                                           Tempera-                                                                           ure rate           Particle                          Example                                                                            Appa-     ration                                                                             ture (kg/                                                                              (ml/                                                                              X-ray                                                                              Particle                                                                            size                              No.  ratus                                                                             Metal salt                                                                          (mol/l)                                                                            (°C.)                                                                       cm.sup.2)                                                                         min)                                                                              analysis                                                                           shape (nm)                              __________________________________________________________________________    1    A   Aluminium                                                                           0.01 350  300 4   Boehmite                                                                           Spindle-                                                                            1500 × 700                           nitrate                      like                                    2    A   Aluminium                                                                           0.053                                                                              400  300 4   "    Acicular                                                                            700 × 100                            nitrate                                                              3    A   Aluminium                                                                           0.01 300  400 4   "    Scale-like                                                                          200                                        nitrate                                                              4    A   Aluminium                                                                           0.01 400  400 4   "    Hexagonal                                                                           300                                        nitrate                      plate-like                              5    A   Aluminium                                                                           0.002                                                                              400  350 4   "    Square                                                                              200                                        nitrate                      plate-like                              6    A   Aluminium                                                                           0.0004                                                                             400  350 4   "    Square                                                                              100                                        nitrate                      plate-like                              7    B   Fe(NO).sub.3                                                                        0.1  400  350 4   α-Fe.sub.2 O.sub.3                                                           Ellipsoidal                                                                          50-100                           8    B   "     0.02 400  350 4   α-Fe.sub.2 O.sub.3                                                           "     30-60                             9    A   Fe.sub.2 (SO.sub.4).sub.3                                                           0.01 400  400 4   α-Fe.sub.2 O.sub.3                                                           Spherical                                                                           20                                                                 + Fe.sub.3 O.sub.4                           10   B   Ferric                                                                              0.02 400  350 6   Fe.sub.3 O.sub.4                                                                   Ellipsoidal                                                                         20-50                                      ammonium                                                                      citrate                                                              11   B   Cobalt                                                                              0.02 400  350 5   Co.sub.3 O.sub.4                                                                   Octahedral                                                                           40-250                                    nitrate                                                              12   B   Nickel                                                                              0.02 400  350 6   NiO  Acicular                                                                            100 × 400                            nitrate                                                              __________________________________________________________________________

What is claimed is:
 1. A process for producing fine metal oxideparticles having a particle size of 20 to 2,000 nm comprising the stepsof heat-treating an aqueous solution of a water-soluble metal salt at atemperature of not lower than 200° C. at a pressure of 250 to 500 kg/cm²for 1 second to 10 minutes to achieve a decomposition reaction of saidmetal salt to form said metal oxide, wherein said aqueous solution ofsaid metal salt is continuously supplied in a tubular reactor in such amanner as to remain in a reaction zone of the tubular reactor at atemperature of not lower than 200° C. and a pressure of 250 to 500kg/cm² for 1 second to 10 minutes, and separating said fine metal oxideparticles, and wherein said metal is selected from the group consistingof copper, barium, calcium, zinc, yttrium, silicon, tin, zirconium,titanium, antimony, vanadium, chromium, manganese, iron, cobalt andnickel.
 2. A process according to claim 1, wherein an alkali or acidicaqueous solution is added to said aqueous solution of said metal salt.3. A process according to claim 1, wherein a reducing gas comprisinghydrogen or an acidic gas containing oxygen is introduced into saidaqueous solution of said metal salt.
 4. A process according to claim 1wherein the temperature is from 300° to 500°.